Recently, there is a demand for high-density optical disks, and the track pitch thereof is being narrowed while the linear density is being increased. In order to achieve a narrow track pitch, it is required to reduce interference with adjacent tracks such as cross-write. Therefore, it is important to conduct tracking control of a beam spot for recording/reproducing with good precision.
As a method for controlling a beam spot, a push-pull tracking system is frequently used. However, this system has problems involved in a shift of an optical axis and a tilt of a disk.
As a method for controlling a beam spot while reducing an error of tracking control even when fluctuations such as a shift of an optical axis occur, a sample servo tracking system is known. According to this system, tracking control is conducted based on a reproduction signal from prepit regions disposed separately on a disk. FIG. 17A schematically shows a conventional configuration of a prepit region on an optical disk adopting the sample servo tracking system.
Referring to FIG. 17A, in a prepit region, each clock pit 2 is disposed on a virtual track center 1. Furthermore, a pair of wobble pits 3 are disposed at positions shifted by a ¼ track from the virtual track center 1. A pair of wobble pits 3 are composed of a first wobble pit 3a and a second wobble pit 3b disposed on different sides of the virtual track center 1. An address pit 4 is formed at a predetermined distance from the second wobble pit 3b along the virtual track center 1.
According to the sample servo tracking system, a tracking error is detected based on the amount of reflected light (reproduction signal) from a pair of wobble pits 3. FIGS. 17B to 17D show reproduction signals in the prepit region shown in FIG. 17A. A section Tc represents a reproduction signal from the clock pit 2, a section Tw1 represents a reproduction signal from the first wobble pit 3a, and a section Tw2 represents a reproduction signal from the second wobble pit 3b. 
The wobble pits 3a and 3b are shifted in opposite directions at the same distance from the virtual track center 1. Therefore, in the case where a beam spot for recording/reproducing passes along the virtual track center 1, a decreased amount V1 of the reflected light in the section Tw1 is equal to a decreased amount V2 of the reflected light in the section Tw2, as shown in FIG. 17B. In the case where a beam spot is shifted to the first wobble pit 3a side, the decreased amount V1 of the reflected light in the section Tw1 is increased, whereas the decreased amount V2 of the reflected light in the section Tw2 is decreased, as shown in FIG. 17C. On the other hand, in the case where a beam spot is shifted to the second wobble pit 3b side, the decreased amount V1 of the reflected light in the section Tw1 is decreased, whereas the decreased amount V2 of the reflected light in the section Tw2 is increased, as shown in FIG. 17D.
As described above, when a beam spot is shifted from the virtual track center 1, a difference is caused between the decreased amounts V1 and V2 of reflected light. According to the sample servo tracking system, by detecting the difference (tracking control signal) between the decreased amounts V1 and V2 of reflected light, tracking control is conducted. According to the sample servo tracking system, all the reflected light from a disk is used, so that tracking control is unlikely to be influenced by a shift of a lens, a tilt of a disk, and the like, which decreases a residual error in tracking control.
JP 4(1992)-301219 A discloses a method for increasing recording density in the above-mentioned sample servo tracking system. According to this method, wobble pits are shared by adjacent tracks. This method can double tracking density, compared with a conventional method.
Furthermore, in a conventional optical disk, the reproduction resolution of a signal is determined substantially by a wavelength λ of the reproduction light and a numerical aperture (NA) of an objective lens, and a pit period of a detection limit is essentially λ/(2/NA). However, it is not easy to shorten a wavelength of reproduction light or increase a numerical aperture of an objective lens. Therefore, various attempts have been proposed for increasing recording density of information by modifying a recording medium and a reproduction method. For example, JP 6(1994)-290496 A discloses a technique of enhancing a reproduction resolution beyond a detection limit determined by a wavelength of reproduction light and a numerical aperture of an objective lens, using a DWDD method. According to the DWDD method, magnetic domain walls move successively by irradiation with a light beam for reproduction, and the movement of the magnetic domain walls is detected. According to this technique, when a reproduction layer that is a first magnetic layer, in which magnetic domain walls move upon being irradiated with a light beam for reproduction, is separated magnetically between respective information tracks, a particularly satisfactory reproduction signal is obtained.
As a method for magnetically cutting off a magnetic layer between information tracks, there is a method for conducting laser annealing between information tracks. However, it takes much time to conduct laser annealing. In order to solve this problem, a method for forming grooves and lands on an optical disk, and separating a magnetic domain wall moving layer by the lands is proposed (see JP 11(1999)-120636 A). Furthermore, in an optical disk using both grooves and lands as recording tracks, a method also is proposed for separating a magnetic domain wall moving layer by using a tilt of an inclined surface of the lands and grooves (see JP 11(1999)-120636 A).
However, in the case where information is recorded/reproduced with respect to a high-density optical disk in accordance with the conventional sample servo tracking system, sufficient tracking accuracy is not ensured. This makes it difficult to realize a high-density optical disk. The problem regarding tracking accuracy becomes particularly serious in the case of an optical disk conducting recording/reproducing in accordance with the DWDD method described in the prior art. This is because the DWDD method allows recording/reproducing to be conducted beyond the limit of a resolution of an optical beam. In a conventional optical disk that does not adopt the DWDD method, the optical resolution controls the recording density. Therefore, when a track pitch is narrowed, reproduction cannot be performed due to crosstalk from an adjacent track. In order to avoid this crosstalk, it is required to increase a track pitch to about 0.67 times of λ/NA. However, according to the DWDD method, recording/reproducing can be performed even when a track pitch is narrowed to about 0.49 times of λ/NA Therefore, tracking accuracy that is much higher than that of a conventional optical disk is required to go along with the enhancement of track density by narrowing a track pitch.
There are the following two problems for achieving such high tracking accuracy in the sample servo tracking system.
1. Due to a tilt of an optical disk, a tracking error occurs, decreasing the tracking accuracy.
2. An amplitude of a tracking control signal is varied between an inner periphery and an outer periphery of a disk, decreasing the tracking accuracy.
The first problem will be described. In tracking control in accordance with the push-pull tracking system used in a conventional optical disk, a DC offset occurs in a tracking control signal by a tilt of a disk and a shift of a lens, and this error decreases tracking accuracy. In contrast, according to the sample servo tracking system, a DC offset is not generated in a tracking control signal due to a tilt of a disk, a shift of a lens, and the like. Therefore, it has been considered in the prior art that the advantage of the sample servo tracking system lies in that a tracking control signal is not fluctuated due to a tilt of a disk, a shift of a lens, and the like. However, in the sample servo tracking system, a tracking error that does not occur in a DC offset of a tracking control signal occurs due to a tilt of a disk, which decreases the tracking accuracy. This phenomenon becomes conspicuous in an optical disk that shares wobble pits between adjacent tracks, as disclosed in JP 4(1992)-301219 A.
FIG. 18A shows a sample servo tracking system in the case where wobble pits are not shared between adjacent tracks, and FIG. 18B shows a sample servo tracking system in the case where wobble pits are shared between adjacent tracks. In an information recording medium in FIG. 18B, the virtual track center 1 includes virtual track centers 1a and 1b having different polarities of reproduction signals from wobble pits. Compared with FIG. 18A, it is understood in FIG. 18B that the wobble pits of adjacent tracks are close to a light beam 5 to cause large interference. Thus, when track density is increased, the interference of wobble pits between adjacent tracks is increased. As a result, a tracking error occurs in the case where a tilt of a disk occurs, which decreases the tracking accuracy. Similarly, the interference of pits before and after wobble pits also causes a tracking error.
As an example, FIG. 19 shows a reproduction signal in the case where an interval between the clock pit 2 and the first wobble pit 3a is insufficient. In this case, the decrease in the amount V1′ of reflected light in the section Tw1 is increased due to the influence of the clock pit 2. Thus, even in the case where a beam spot scans the virtual track center 1, a tracking control signal (V1′−V2) does not become 0, and exact tracking control cannot be conducted.
Next, the second problem will be described in which an amplitude of a tracking control signal is varied between an inner periphery and an outer periphery of a disk, and the tracking accuracy is decreased.
FIGS. 20A to 20C show how a reproduction signal is changed due to the length of a prepit. In the case where a prepit is too short, an amplitude of a signal is too small to conduct tracking control, as shown in FIG. 20A As a prepit becomes longer, an amplitude of a signal is increased as shown in FIG. 20B. When a prepit becomes longer compared with the state shown in FIG. 20B, an amplitude is decreased slightly to form a flat portion, as shown in FIG. 20C.
When an amplitude of a reproduction signal from the wobble pits is changed depending upon the position on a disk, an amplitude of a tracking control signal is varied depending upon the position on a disk, which decreases the reliability of tracking control. In order to avoid this problem, a method is proposed for prescribing a wobble pit to be longer than a spot diameter to form a flat portion in a reproduction signal, and using a signal of this portion to obtain a stable tracking control signal (see JP 5(1993)-73929 A). However, if a wobble pit is made longer, the prepit region becomes longer, so that linear density is decreased.
Thus, in order to achieve high density in an optical disk adopting the sample servo tracking system, the size of a prepit in a prepit region and the interval between prepits are important.
In particular, according to the DWDD method, data is recorded onto a recording track cut off magnetically from an adjacent track, and a gradient of a heat distribution is used to conduct enlargement reproduction. Therefore, when tracking offset occurs during recording and reproduction, reproduction characteristics are degraded remarkably to cause an error. In order to conduct reproduction without an error, it is required to suppress a tracking error within ±0.04 μm. In an optical disk that is a medium to be replaced, it is very difficult. to suppress a tracking error (including those occurring due to convertibility and vibration between apparatuses) within ±0.04 μm.
The above-mentioned sample servo tracking system is excellent, in which a tracking error is unlikely to occur due to a tilt of a disk, a shift of a lens, and the like. However, it is very difficult to decrease a tracking error under various conditions even by using this system. In the sample servo tracking system, variations in an amplitude of a tracking control signal in a disk and a detection error of a tracking position occurring from a disk tilt become main factors of a tracking error. Furthermore, the margin of a tracking error needs to cover factors such as a control residual error involved in tracking control, a tracking error occurring due to vibrations, and a detection error at a tracking position.
In an ordinary optical disk, there are a control residual error of about ±0.015 μm and a tracking error of ±0.02 μm caused by vibrations. In the DWDD method in which a tracking error margin is very small, i.e., ±0.04 μm, a detection error of only about ±0.005 μm of a tracking position remains. Therefore, a detection error of a tracking control signal in the sample servo tracking system, which has not been a problem in the prior art, also becomes a serious problem. Since a track pitch of a high-density optical disk is about 0.5 to 0.6 μm, a detection error of ±0.005 μm becomes about 1% of a track pitch. Therefore, in order to realize a high-density optical disk, it is required to realize the above-mentioned tracking position detection error in the smallest possible servo region.